Conformational analysis of cyclo(Phe-Ser) by UV–UV and IR–UV double resonance spectroscopy and ab initio calculations

We present the resonant two-photon ionization (R2PI) spectra as well as the UV–UV and IR–UV double resonance spectra for the cyclic dipeptide Phe-Ser. The R2PI spectrum shows five strong transitions in the region of 37 500–37 900 cm . By performing UV–UV double resonance spectroscopy, we distinguished 5 different conformers. For each of these conformers, the origin is the most intense transition. In addition, we performed IR–UV double resonance measurements in the region 3200–3800 cm 1 to analyse the NH and OH modes of each conformer. We compared the measured IR spectra to frequencies from ab initio calculations to assign each conformational structure. We found two structures in which the hydroxyl group of the serine residue forms a strong hydrogen bond with the carboxyl group of the same residue. One structure shows only a weak hydrogen bond and for the remaining two structures, the hydroxyl group is ‘free’.